The Plain Old Telephony Service (POTS) has since its origin been based on remote power feeding also called battery feed. The phone set at the subscriber premises is powered by using the current flowing in the subscriber loop provided by a DC power supply in the central office. Using remote power feeding the phone also works in case of mains outage, a feature sometimes called “life-line”.
The power needed to feed a subscriber line depends on the loop length. Longer subscriber lines need a higher voltage level than shorter lines and can be up to 60 Volt DC. This level has been selected to be high enough to get through kilometers of subscriber lines but still low enough to be safe.
Excess power that is not transmitted along the subscriber line (due to impedance mismatch) is dissipated as heat at the subscriber line interface unit (line card) and is referred to as power dissipation.
Different solutions to provide power feeding to subscriber lines are known. One solution is to use a DC power supply (today often a DC/DC converter) for each subscriber line. By using one DC power supply per subscriber line, each subscriber line can be fed by a voltage level just above what it needs to operate and the power dissipation is minimized.
One disadvantage with this solution is that it is expensive. Another disadvantage is that the design becomes very complex for line cards comprising a large number of subscriber line interface circuits (SLIC). All DC/DC converters also consume a large area on the line card.
Yet another disadvantage is that when using a large number of DC/DC converters on the same line card, the converters add electromagnetic interference when they independently regulate the different subscriber lines.
Addressing some of these disadvantages, a number of other solutions have been proposed.
One solution is disclosed in U.S. Pat. No. 6,760,430. In this patent, a common voltage regulator feeds a number of subscriber line interface circuits (SLIC) or line drivers. The common voltage regulator has to provide a voltage level sufficiently high so that all subscriber lines including the longest subscriber line can operate. This means that for each subscriber line that is shorter than the longest one there is excess power that dissipates as heat from the line card.
Another solution is disclosed in U.S. Pat. No. 6,351,534. In this patent, subscriber lines are grouped according to their loop lengths. In one embodiment a group with the loop length 0 to 300 meters is connected to one line card and another group with the loop length 300 to 1000 meters is connected to another line card and so on. The line cards are mounted in a rack. Each group is fed by a power supply mounted on each line card with a sufficient voltage level sufficient to operate all subscriber lines in that group.
This solution reduces power dissipation compared to a solution where all subscriber lines (independent of loop lengths) are fed from the same voltage regulator as in U.S. Pat. No. 6,760,430.
One disadvantage with the solution in U.S. Pat. No. 6,351,534 is that it uses fixed range groups. This will result in a number of limitations when it comes to reconfiguring the rack comprising the line cards.
For example, when connecting a new 200 meters long subscriber line and if the line card configured for the range 0 to 300 m is full, it is necessary to add a new card for that range instead of using an unused subscriber line interface circuit on any of the other line cards. Another example, if one subscriber in the range 300 m to 1 km ends his/her subscription, the free subscriber line interface circuit is left unused until a new subscriber within the same range is identified.
It has been observed that the lengths in a set of subscriber lines connected to a central office seldom are evenly distributed. The normal case is that the line lengths are concentrated around certain ranges which in turn can vary depending on where the central office is located (such as rural or highly populated areas). Moreover, the ranges can also differ very much from country to country. Therefore, using a solution with fixed range groups there may also be a need to make additional market configurations and/or adaptations.